Abstract

This SRFI proposes an extension to the cond syntax to allow a more
general clause, one that allows binding the results of tests as in the
=> clauses and user-defined meaning of the success & failure of tests.

Rationale

The present set of cond clauses is based on simple boolean testing. It
is prohibitively inexpressive in that the condition part of a cond
clause that uses => may pass only a single value to the receiver, and
it enforces a semantics whereby #f implies failure of the condition.
Programmers frequently use different tokens to imply failure, such as
in R5RS's I/O readers which return a distinguished 'EOF object' to
denote failure, and a successful condition may produce more than one
useful value. This simple extension allows any meaning of 'failure' to
be assigned on a per-clause basis, and it also allows the condition to
return multiple values to be passed to the receiver.

Specification

The <cond clause> production in the formal syntax of Scheme as written
by R5RS in section 7.1.3 is extended with a new option:

<cond clause> --->
...
| (<generator> <guard> => <receiver>)

where <generator>, <guard>, & <receiver> are all <expression>s.

Clauses of this form have the following semantics: <generator> is
evaluated. It may return arbitrarily many values. <Guard> is applied
to an argument list containing the values in order that <generator>
returned. If <guard> returns a true value for that argument list,
<receiver> is applied with an equivalent argument list. If <guard>
returns a false value, however, the clause is abandoned and the next
one is tried.

Examples

This port->char-list procedure accepts an input port and returns a list
of all the characters it produces until the end.

Consider now a hypothetical table-entry procedure that accepts two
arguments, a table (perhaps a hash table) and a key to an entry that
may be in the table; it returns two values: a boolean that denotes
whether or not an entry with the given key was in the table and, if it
was, the value associated with the key. Also, a hypothetical proj0
combinator (projection of argument 0) returns its 0th argument and
ignores all others. One might conditionally branch to a certain body
of code if the table contains the desired entry like so with the new
type of cond clause:

Implementation

The entirety of a syntax transformer for the new cond syntax is given
here. It uses an auxiliary macro, cond/maybe-more, to simplify the
construction of if expressions with or without more cond clauses. The
code is in the public domain.

Copyright

Copyright (C) 2004 Taylor Campbell. All rights reserved.

Permission is hereby granted, free of charge, to any person obtaining
a copy of this software and associated documentation files (the
"Software"), to deal in the Software without restriction, including
without limitation the rights to use, copy, modify, merge, publish,
distribute, sublicense, and/or sell copies of the Software, and to
permit persons to whom the Software is furnished to do so, subject to
the following conditions:

The above copyright notice and this permission notice shall be
included in all copies or substantial portions of the Software.

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